Graphene electrodes for organic metal-free light-emitting devices

Graphene electrodes for organic metal-free

light-emitting devices

Nathaniel D Robinson, Ludvig Edman and Manish Chhowalla

Linköping University Post Print

N.B.: When citing this work, cite the original article.

Original Publication:

Nathaniel D Robinson, Ludvig Edman and Manish Chhowalla, Graphene electrodes for

organic metal-free light-emitting devices, 2012, Physica Scripta, (T146), 014023.

http://dx.doi.org/10.1088/0031-8949/2012/T146/014023

Copyright: Institute of Physics

http://www.iop.org/

Postprint available at: Linköping University Electronic Press

http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-76203

http://dx.doi.org/10.1088/0031-8949/2012/T146/014023

http://www.iop.org/

http://urn.kb.se/resolve?urn=urn:nbn:se:liu:diva-76203

Graphene electrodes for organic metal-free

light-emitting devices

Nathaniel D Robinson1, Ludvig Edman2 and Manish

Chhowalla3

1 The Transport and Separations Group, Department of Physics, Chemistry andBiology, Linkoping University, SE-58183 Linkoping, Sweden2 The Organic Photonics and Electronics Group, Department of Physics, UmeaUniversity, SE-90187 Umea, Sweden3 Department of Materials Science and Engineering, Rutgers University, 607 TaylorRoad, Piscataway, New Jersey 08854, USA

E-mail: [email protected]

Abstract. In addition to its fascinating electrical and mechanical properties,graphene is also an electrochemically stable and transparent electrode material.We demonstrate its applicability as both anode and cathode in a light-emittingelectrochemical cell (LEC), an electrochemical analog to a polymer organic light-emitting diode (POLED). Specifically, we summarize recent progress in carbon-basedmetal-free light-emitting devices enabled by chemically-derived graphene cathodeson quartz and plastic substrates, and explain the advantages of using LECs inmanufacturing large-area devices.

1. Introduction

“Organic electronics” has long been defined to encompass devices that include some

electronically-active organic component, i.e. nearly all of todays “organic” light-

emitting devices include two inorganic electrodes to contact an organic emissive layer.

Still, the attractive properties of organic materials, specifically, the relatively small

cost of solution-based manufacturing (via printing and coating), mechanical flexibility

and ability to tune color and other parameters through chemistry, have motivated a

tremendous investment in the development of new and improved organic materials for

the realization of true all-organic devices. Here, we show that the advent of graphene,

with its combination of transparency, conductivity and chemical stability, opens the

possibility of metal-free all-organic light-emitting devices. [1, 2]

We review our recent progress regarding the use of chemically derived graphene

(CDG) as a transparent electrode in polymer light-emitting electrochemical cells (LECs).

A schematic of the most practical device demonstrated to date, in which a blend of a

light-emitting polymer (LEP) and an electrolyte is sandwiched between a CDG cathode

and conducting polymer anode, is shown in figure 1.

Graphene electrodes for organic metal-free light-emitting devices 2

anode

p-n junction

graphene cathodesubstrate

p-doped region

n-doped region

}

}

PEDOT

PSS- H+Super Yellow

PEO

OC10H21

OC10H21

OC10H21

OC10H21

x

zy

S

O O

n

On

O–S

F

F

F

O

O K+

SO3- H+

Figure 1. Schematic of the metal-free LEC as published in Matyba et al. [1] Copyright2010 ACS. Reused with permission.

1.1. Transparent, metal-free electrodes

The vast majority of solid-state light-emitting devices require a transparent electrode for

operation, as do displays (e.g. liquid crystal displays, LCDs) and photovoltaic devices

(solar panels). “Organic” light-emitting devices have long used indium tin oxide (ITO)

as a highly-transparent, conducting anode [3, 4]. For the case of polymer light-emitting

devices, a metal cathode with a low work function has also been necessary for the

attainment of facile electron injection and efficient device operation. [5]

The development of scalable processes for manufacturing large-area transparent

electrodes based on CDG represents a breakthrough in this field from a processing

perspective, and more importantly a materials perspective. The demand for ITO and

similar materials is increasing as consumer electronics become more popular, imparting

pressure on both the supply (particularly of In) and recycling technology (which is

currently insufficient for handling the large volume of these materials incorporated

into electronic devices). Using carbon-based graphene solves both problems, as the

raw material is seemingly endless on this planet and recycling of carbon-based metal-

free electronic devices can be as simple as burning them to recover the energy in the

materials, yielding little other than water and carbon dioxide.


(a) Graphene

n

(b) poly(para-phenylenevinylene) (PPV)

Figure 2. Graphene (left) is an example of an ordered two-dimensional π-conjugatedsystem. PPV (right) is a one-dimensional π-conjugated system when diluted in asolvent, but intrachain and interchain interactions increase the dimensionality of theπ-system in solid films.

1.2. Organic electronics

The current effort to develop electronic components such as lamps, displays, sensors,

solar cells and control systems based on organic molecules and polymers for currently

“inactive” space is tremendous. Billions of dollars of research and development are

pushing to bring new functionality to areas such as consumer packaging and the walls

of your home and office. During the past 20 to 30 years, tremendous progress has been

made in advancing materials and devices to meet this challenge.

One category of organic electronics, in which polymeric (plastic) materials are

primarily used, has several particular advantages. Like nearly all organic electronic

materials, polymers are synthetic, so they can be customized and tailored to include

a wide variety of functionality. However, polymers are also often solution processable,

meaning that they can be dissolved or suspended in a solvent, forming an “ink-like”

material that can be coated or printed with a variety of techniques. This makes polymer

electronic materials very easy, and therefore inexpensive, to process.

The conductivity in these polymers comes from an extended π-structure along the

backbone. A series of alternating single and double C-C bonds causes this so-called

π-conjugation. Just as graphene has a two-dimensional π-system, giving rise to its

incredible electronic properties, a π-conjugated polymer has a similar one-dimensional

system along its backbone. Figure 2 shows the two structures side-by-side. Note that

in the bulk of a solid film, where the conjugated polymer is intertwined with both

itself and neighboring polymer chains, intrachain and interchain interactions increase

the dimensionality of the conjugated system. Many conjugated polymers are highly

fluorescent, and the emission color (and other characteristics) of these LEPs can be

tailored during synthesis by, e.g., adding various side-chains to the polymer backbone.

Derivatives of, and copolymers containing, poly(para-phenylene vinylene) (PPV) are

commonly used in emissive applications, but many other classes of LEPs are also

available. [6]


Substrate (quartz or PET)

Cathode (Ca/Al or LiF/Al)

Light-emitting polymer

Anode (ITO or graphene)

(a) POLED


Cathode (Al)

LEP and polymer electrolyte

Anode (ITO or graphene)

(b) “Sandwich” LEC


LEP and polymer electrolyteElectrodes (graphene)

(c) Planar LEC

Figure 3. Architectures of a POLED, “sandwich” LEC, and planar LEC.

1.2.1. Polymer (organic) light-emitting diodes (POLEDs) The polymer light-emitting

electrochemical cell (LEC) is an electrochemical analog of the polymer organic light-

emitting diode (POLED) [7]. The devices are similar in structure, but the operational

mechanisms differ tremendously, particularly due to the mobile ions in the LEC. We

will describe the operation of LECs in context with POLEDs, simply because the latter

is much more widely understood.

A minimalistic POLED consists of a thin layer (<100 nm) of a LEP (e.g. a PPV)

sandwiched between two metal electrodes, as shown in figure 3a. If the HOMO of the

LEP is energetically near the Fermi level of the anode (positive electrode), and the

LUMO of the LEP is near the Fermi level of the cathode, then applying a potential

between the metal electrodes can cause injection of a hole into the LEP at the interface

with the anode and an electron at the cathodic interface. The hole and electron are

then transported by a combination of electromigration and diffusion until they meet and

form a bound electron-hole pair (an exciton), which can decay under the emission of

light. As previously mentioned, at least one of the metal electrodes must be transparent

if the light generated is to escape the device.

ITO is commonly used as an anode (often with a planarizing layer of the p-doped

conjugated polymer PEDOT:PSS) in POLEDs, as the Fermi level of ITO is sufficiently

near the HOMO of most LEPs. Ca or a bilayer of LiF and Al are typical cathode

materials because their high Fermi level (corresponding to a small work function)

matches the LUMO of the LEP. Working with low-work-function cathode materials

like Ca is, however, a challenge, as they are very reactive with oxygen and water.

Furthermore, there are no known transparent materials that work well as a cathode in

POLED devices. However, graphene was predicted and has been demonstrated[8] to

work as an anode in small-molecule OLED devices.

1.2.2. Electrochemical doping of polymers The electronic properties of inorganic

semiconductors can be tuned via blending or implanting small amounts of “dopant”


materials with differing valence into the host semiconductor, which effectively

donate/accept an electron to/from the semiconductor host. This chemical doping results

in the formation of conducting doped regions, which, for instance, can be utilized for

the realization of high-speed transistors that are the basis for the microprocessors and

memory chips in today’s personal computers.

Conjugated polymers can also be chemically doped by adding appropriate oxidizing

or reducing agents to the polymer, resulting in a “permanently” doped material.

Alternatively, when the conjugated polymer is in contact with mobile ions and an

electrode, it can be doped electrochemically, in situ. The ions can come from a salt

added directly to the polymer if the polymer can dissolve it, but usually a solid-state

solvent such as polyethyleneoxide (PEO) is required. The doping process can either be

of p-type:

P + X− � P+X− + e− (1)

where P represents the undoped polymer, X− represents an anion (from the salt M+X−),

P+ represents (a segment of) p-doped polymer and e− represents an electron; or of n-

type:

P + M+ + e− � M+P− (2)

where M+ represents a cation from the dissolved salt and P− represents (a segment of)

n-doped polymer.

As with inorganic semiconducting materials, doping a polymer semiconductor can

change its conductivity several orders of magnitude. [9, 10] The polymers also typically

change color [11, 12] and size, [13, 14] and their surface chemistry alters [15, 16, 17]

during the doping process. In general, many “field-effect” (ion-free) polymer devices

have electrochemical analogs. [10].

1.3. The light-emitting electrochemical cell (LEC)

Blending an ion-solving material such as PEO and a salt (e.g. KCF3SO3) with the LEP,

and fabricating the same sandwich structure previously described creates an LEC. The

mobile ions serve several functions. First, they assist charge injection. When a potential

is applied between the electrodes, the anions and cations electromigrate (drift) toward

the anode and cathode, respectively. The metal electrode is impermeable to ions, so the

ions congregate in a dense layer of high charge density at the interface, with an equal and

opposite charge density in the metal electrode itself, forming a so-called electric double-

layer. This double-layer has a thickness on the order of the Debye screening length which

is typically around 1 nm, and which is short enough for tunneling of electrons through

the double-layer to be significant. Thus, the difference in energy between the LEP’s

HOMO and LUMO and the Fermi level of the anode and cathode are no longer critical

to the operation of the device (although they can still affect the device [18, 19]). In other

words, efficient LECs can be made with a much wider variety of electrode materials than

POLEDs. In fact, the same material can often be used for both electrodes. [20, 21, 22]


Once charge injection into the LEP layer occurs, the introduction of mobile

electrons/holes in combination with the existence of mobile ions allows electrochemical

doping to occur (see (1) and (2)). Thus, the LEP becomes p-doped near the anode and

n-doped near the cathode. This converts the polymer to a (relatively good) conductor,

extending the effective anode and cathode into the LEP. This process continues and

the doped regions expand toward one another until they meet, forming a p-n (or p-i-

n) junction. At this point, the device turns on, as holes and electrons can meet and

recombine in the narrow strip of undoped material between the two doped regions.

One desirable consequence of this doping process is that LEC operation is relatively

independent of the thickness of the active layer. Thicker devices require more time to

turn on, but can still operate at potentials at or slightly above the bandgap of the LEP

[23]. POLEDs, on the other hand, are field-dependent, which means that the voltage

required to operate the device increases with the thickness of the LEP layer. Low-

voltage devices are desirable, particularly for battery-powered portable systems, while

“thick” layers (> 500 nm) are relatively easy to manufacture with coating and printing

equipment and processes. Thinner layers are more susceptible to pinhole defects that

cause short-circuits, rendering a device inoperable. This challenge becomes particularly

important with increasing device area, where even extremely low defect densities (e.g.

those found in the silicon semiconductor industry) become unacceptable.

The device thickness can be taken to extremes. For example, rather than making a

sandwich structure, one can make a planar device like the one shown in figure 3c. In this

device, the gap between the electrodes (which are usually the same material) can be up

to several mm [24, 21, 22, 19, 25]. This allows the doping progression described above to

be followed over time due to the sharp contrast between the photoluminescent undoped

LEP and the doped LEP where the photoluminescence is quenched. [24] Furthermore,

the surface potential within the device can be measured via scanning kelvin probe

microscopy (SKPM). [26, 27, 28] See Robinson et al. [22, 25] and Matyba et al. [28]

for measurements, analysis and discussion of the turn-on process in wide-gap devices.

1.3.1. Metal-free LECs Several opportunities arise because LECs are compatible with

a wider range of electrode materials than OLEDs. For example, high-work-function

materials such as graphene can be used as both the anode and cathode, eliminating

the need for both the ITO and the reactive metal found in the vast majority of

OLED devices. [1] Other carbon allotropes such as carbon nanotubes, have also been

demonstrated as transparent electrodes in polymer LECs. [29]

2. Preparation of electrodes

A summary of the preparation of the electrodes is provided here. For details, we refer

to several previous publications. [1, 2]


Figure 4. (a) Process used for coating CDG films on a silicon oxide surface (i–iv)and then transferring the film to PET (v–ix). Examples of the resulting films a 6inch substrate (b) and 12 inch substrate (c). Copyright ACS 2010. [32] Used withpermission.

2.1. Quartz substrates

Transparent CDG electrodes were fabricated on quartz substrates from graphene oxide

(GO) flakes exfoliated from graphite powder using a modified Hummer’s method [30].

The GO flakes, suspended in water after exfoliation, were first vacuum-filtered onto

cellulose paper filters [31] and then transferred to quartz substrates. The GO film was

then reduced in an Ar/H2 environment and annealed at 1000 ◦C in vacuum overnight.

2.2. Transfer to PET

Once fabricated, the CDG films described above can be transferred to flexible

polyethylene terephthalate (PET) substrates, yielding transparent, flexible electrodes.

The process is relatively simple. A thin layer of poly(methyl methacrylate) (PMMA)

is spin-coated onto the CDG film and baked. The CDG film can then be released from

the quartz substrate by submersing the entire stack into a sodium hydroxide (NaOH)

solution. Once delaminated, the PMMA/CDG film is placed onto the PET substrate

and acetone used to remove the PMMA. A schematic of this processcan be found in

figure 4a (parts v–viii) Photographs of the resulting use of this process for obtaining

large substrates can be seen in figure 4b–c.

3. Characterization of electrodes

3.1. Optical and electronic characteristics.

The optical transparency of CDG electrodes fabricated with the processes described

above is between 50% for thick films to well over 90% for thin films. [32] The sheet


Figure 5. (a) Upper panel: cyclic voltammogram recorded using a thin film of CDGas the working electrode. Lower panel: cyclic voltammogram recorded using CDGcoated with a thin film of Super Yellow. (b) AFM topography image of a CDG filmon quartz (c). A micrograph of the same CDG film. Figure copyright ACS 2011. [2]Used with permission.

resistance of the films used in LECs was measured to be approximately 5kΩ/� on both

the quartz [1] and PET [2] substrates.

3.2. Roughness

CDG electrodes manufactured using the above-described method are relatively rough

(compared to atomically flat graphene films). Figure 5b shows an AFM measurement

of a CDG film on quartz. [2] Fortunately, the LEC is particularly robust to electrode

roughness and variations in the thickness of the LEP layer. Thus, it should prove

significantly simpler to use CDG electrodes when manufacturing LECs than POLEDs.

3.3. Electrochemical stability

Since LECs include mobile ions, and therefore the opportunity for electrochemical

reactions, the electrochemical stability of the electrode materials chosen is critical.

A standard technique for measuring the electrochemical reactivity of a material is

cyclic voltammetry, where the potential applied to the material to be tested is cycled

linearly over a potential range in a three-electrode electrochemical cell. Figure 5a

shows the result of a CV experiment performed at 50 mV/s, in which the CDG

electrodes (on quartz) were submersed in a solution of 0.1 M tetrabutylammonium


hexafluorophosphate (TBAPF6) in anhydrous acetonitrile. After cycling the potential,

measured versus a Ag quasi-reference electrode, bis-(g-cyclopentadienyl)iron(II)

(ferrocene, Fc) was added to the cell and a calibration sweep executed. The potentials

shown in figure 5a are reported with respect to the redox potential of the Fc/Fc+ system.

[2]

Notice that the CDG electrode (upper voltammogram) shows very little

electrochemical activity in the potential range of interest. When a layer of Super Yellow

is spin-coated onto the CDG electrode (as in the preparation of the CDG/Super Yellow

POLED), it is clear that the Super Yellow is both electrochemically oxidized (p-doped,

at approximately 0.7 V vs. Fc/Fc+) and reduced (n-doped, at approximately -2.3 V vs.

Fc/Fc+) in this same potential window.

It should be noted that the CDG studied here (and likely graphene in general)

shows superior electrochemical stability than even PEDOT:PSS or ITO. PEDOT:PSS

is primarily suitable only as an anode in this type of device, as its conductivity decreases

several orders of magnitude when electrochemically reduced (dedoped). Similarly, ITO

tends to undergo a variety or reduction processes causing it to change color when used

as a cathode. As demonstrated by the CV measurement in figure 5a and the devices

described below, CDG is suitable as both anode and cathode for LECs (and likely many

other electrochemical devices). This should not come as a surprise, as the electrode is

expected to be chemically similar to the surface of carbon black electrodes, which have

been used extensively in electrochemistry for decades.

4. Fabrication of devices

A brief summary of the device fabrication is presented here. Details can be found in

the original articles [1] and [2].

4.1. Light-emitting polymer layer

The active layer of each of the LEC devices presented here was fabricated from

a cyclohexanone solution of “Super Yellow” (LEP from Merck), the salt potassium

trifluoromethanesulfonate (KTf) and the ion-dissolving material PEO. This solution

was drop-cast onto the CDG substrate and dried on a hotplate at 360 K for 12 h,

resulting in a 1–2 μm thick film.

The active layer of the CDG POLED was spin-cast from a solution of Super Yellow

in cyclohexanone, resulting in a LEP film approximately 70 nm thick.

4.2. Stenciling PEDOT:PSS electrodes

A 2 by 4 array of conducting electrodes was patterned on top of the active layer

as follows: A stencil (mask) created using cellophane tape was placed on top of the

LEP film. The device and stencil were then placed on a hotplate at 360 K for more


than 10 minutes. Clevios S V3, a viscous aqueous suspension of a blend of poly(3,4-

ethylenedioxythiophene) and polystyrene sulfonic acid (PEDOT:PSS) commercially

available from H.C. Starck, was spread over the device through the stencil using the

edge of a glass microscope slide. After removing the stencil, the device was again baked

on the hotplate at 390 K for 12 h.

To assist electrical contacting of the PEDOT:PSS electrodes, a Ag/AgCl paste was

applied to the ends of the polymer electrodes.

5. Devices

5.1. POLED with graphene anode on quartz

First, we have demonstrated a simple POLED with a CDG anode, Super Yellow active

layer, and Ca capped with Al (each layer thermally evaporated, 50 nm thick) cathode.

This device performed reasonably well, although the relatively rough graphene electrodes

caused microshorts (small short-circuits) within the film that resulted in visible dark

spots and an excessive current through the device. The device turned on at an applied

potential of 7 V and emitted more than 200 cd/m2 at 23 V and a quantum efficiency

of 1.5 cd/A. This result confirms that CDG can be a viable low-cost replacement for

ITO as the anode in POLEDs, although it does not match the results presented for a

small-molecule OLED by Wu et al. [8]

5.2. LEC with graphene cathode and PEDOT:PSS anode on quartz

The device shown schematically in figures 1 and 3b is an improvement over the analogous

POLED in terms of the materials involved (no metals required), the fabrication process

(all materials are solution-processed), and the electronic and optical efficiency of the

device. As shown in figure 6a and 6b, the device emits bright light through both the

anode and cathode, as both are relatively transparent.

As previously mentioned, the mobile ions in polymer LECs assist charge injection

and allow electrochemical doping to form a p-n junction in situ. This means that the

device turns on when a potential corresponding to the bandgap of the polymer is applied.

Figure 7b shows very clearly that the device turns on at approximately 2.8 V, which is

close to the reported bandgap of Super Yellow of approximately 2.4-2.5 V [33].

5.3. LEC with graphene cathode and PEDOT:PSS anode on PET

The LEC is equally robust when fabricated on a CDG electrode on PET rather than

quartz. Figure 8 shows the current and measured brightness from a Super Yellow device

with PEDOT:PSS anode and CDG on PET cathode. The optoelectronic performance

is comparable with the data shown in figure 7a for the device on a quartz substrate,

but is also flexible, as visible in the photograph shown in figure 9. Here, the device is

wrapped around a test tube containing a suspension of CdSeYZnS quantum dots, which


(a)

(b)

Figure 6. A metal-free solution-processed LEC on quartz observed during lightemission viewed through the CDG cathode (a) and PEDOT:PSS anode (b). CopyrightACS 2010. [1] Used with permission.

absorb the yellow light transmitted through the CDG electrode into the tube and emit

red light. The yellow light visible in figure 9 is transmitted through the PEDOT:PSS

anode.

5.4. LEC with graphene anode and cathode

As described previously, LECs can be manufactured with a planar geometry and with

the same material as the anode and cathode. To demonstrate this capability with CDG,

we created a planar device with identical CDG anode and cathode. First, a 2 mm-wide

stripe of a CDG film was removed by mechanically scraping it away with a scalpel. This

substrate was then coated with the previously-described blend of Super Yellow, KTf

and PEO and dried on a hotplate, resulting in a device similar to the schematic shown

in figure 3.

The device was illuminated by UV light in an otherwise dark environment, allowing

the photoluminescence of undoped Super Yellow to be detected by a camera (see figure


0

200

400

600

800

0 5 10 15 20

0

100

200

300

Current

Cu

rren

t d

ensi

ty (

A/m

2 )

Voltage (V)

Brightness Bri

ghtn

ess

(cd

/m2 )

(a)

2 3 4 5

0.1

1

10

Brightness

Bri

ghtn

ess

(cd

/m2 )

Voltage (V)

(b)

Figure 7. Optoelectronic performance of an LEC with CDG cathode (on quartz) andPEDOT:PSS anode. (a) Current (solid symbols) and brightness (open symbols) as afunction of applied potential. (b) The same brightness data presented on a logarithmicscale near the turn-on voltage. Copyright ACS 2010. [1] Used with permission.

0

300

600

900

1200

0 5 10 15 200

1

2

3

4

5 Current

Cur

rent

(mA

)

Voltage (V)

Brightness

Brig

htne

ss (c

d/m

2 )

Figure 8. Optoelectronic performance of an LEC with PEDOT:PSS anode and CDGon PET cathode. Brightness measured on the CDG/PET side of the device. CopyrightACS 2011. [2] Used with permission.


Figure 9. Photograph of a flexible LEC with PEDOT:PSS anode and CDG on PETcathode. The device was wrapped around a test tube containing a suspension of red-emitting (CDSe/ZnS) quantum dots. Copyright ACS 2011. [2] Used with permission.

10). When a potential (20 V) is applied, dark p- and n-doped regions are clearly visible,

stemming from the anode (left electrode) and cathode (right electrode), respectively.

As described in the introduction, the doped regions grow (images II-IV) until they meet

(V), at which point a thin p-n junction is formed and electroluminescence begins. The

electroluminescence from the p-n junction is more clearly visible in image VI, where the

p-n junction is fully formed and the UV illumination has been removed.

6. Prospect for the future

6.1. Encapsulation

As with POLEDs, the single most important impediment to commercialization of large-

area LEC lighting devices is the lack of inexpensive, flexible and sufficiently impermeable

barriers for water and oxygen. The polymers used in both LECs and POLEDs are

extremely reactive when n-doped, and are either reduced by water and/or oxygen

(removing the electron from the system) or otherwise chemically react with the water

and oxygen, often destroying the polymer. Thus, the various barrier systems that are

now being developed using a wide variety of strategies, including composites of ceramic

and plastic or metal and plastic, will be critical for the commercialization of flexible

polymer-based lighting devices.

The chemical impermeability of graphene itself offers potential protection for

polymer electronic devices as well. However, the CDG processes that are currently

used to make large-area transparent electrodes produce films that are far from “perfect”


Figure 10. A planar LEC with CDG as both anode and cathode, separated by 2 mm.The device is operated at 20 V and 380 K. Illuminating the device with UV light inan otherwise dark environment allows the growth of the doped regions (dark) from theCDG electrodes (interface identified by vertical white dotted lines) to be visualized.The Roman numerals indicate the time (measured from application of potential) eachphotograph was recorded: (I) 0 s, (II) 2 s, (III) 4 s, (IV) 32 s, (V) 68 s, (VI) 78 s.Copyright 2011 ACS. [2] Used with permission.

graphene sheets, but rather patchworks of interconnected graphene flakes with gaps and

holes that will likely allow oxygen and water permeation.

6.2. Opportunities for larger-scale production and operation

The electrodes and devices presented here have the potential to be used in continuous

large-scale high-volume manufacturing processes such as roll-to-roll printing and coating.

Although the high-temperature reduction/annealing process (described above) to make

the CDG electrodes is not compatible with the PET substrate, chemical reduction of

GO films in room-temperature solution, or transfer of graphene films from thermally

stable substrates to flexible foils in a roll-to-roll process have been demonstrated. [34]

The latter is currently under commercial development by Samsung. We reiterate that

the mobile ions in LECs make them particularly suitable for printing and coating LECs

onto graphene substrates because the work function (Fermi level) and roughness of the

electrodes have minimal impact on device performance and relatively thick LEP films

can be used, alleviating the challenge of making uniformly-thin and defect-free films

over large areas. The metal-free sandwich device described above should be possible to

manufacture in a roll-to-roll coating process today. In the near future, LECs with CDG

as both anode and cathode will almost certainly be achievable by laminating the second


electrode (backed by e.g. PET) onto a PET-CDG-LEP stack in a roll-to-roll process.

In either case, we hope to see products such as large-area lighting panels, illuminated

posters and electric wallpaper in the not-so-distant future.

6.3. Performance improvements

The gap between the development of OLEDs and LECs can largely be attributed to the

disappointing device lifetimes that persisted almost a decade after their invention.[33]

Furthermore, the combination of mobile ions and electrons and complications involving

electrochemical side-reactions impeded the understanding of these devices’ operation,

and hence limited the improvements that could be made in regards to performance.

However, through careful and thorough study of these devices, a clearer picture of both

the operating and failure mechanisms in LECs is emerging, allowing rapid improvement

in the device lifetime and performance. [35, 36] In the future, we see no fundamental

reason why LECs cannot match the lifetime and efficiency of POLEDs.

Many of the strategies proposed for improving the efficiency of POLEDs, such as

designed light out-coupling and the use of phosphorescent emitters to harvest the so-

called “triplet states” are directly applicable to LECs. The opportunities with LECs,

in which one can adjust the LEP, solvent and ions, may in fact exceed the potential for

OLEDs.

Acknowledgments

Acknowledgements NDR acknowledges funding from the Swedish Research Council

(Vetenskapsradet). LE is a “Royal Swedish Academy of Sciences Research Fellow”

supported by a grant from the Knut and Alice Wallenberg Foundation, and also

acknowledges support from the Swedish Research Council (Vetenskapsradet) and

Kempestiftelserna. The three-dimensional image in figure 1 was commissioned from

www.planet11.com.

References

[1] Piotr Matyba, Hisato Yamaguchi, Goki Eda, Manish Chhowalla, Ludvig Edman, and Nathaniel DRobinson. Graphene and mobile ions: The key to all-plastic, solution-processed light-emittingdevices. ACS Nano, 4(2):637–642, 2010. PMID: 20131906.

[2] Piotr Matyba, Hisato Yamaguchi, Manish Chhowalla, Nathaniel D Robinson, and Ludvig Edman.Flexible and metal-free light-emitting electrochemical cells based on graphene and pedot-pss asthe electrode materials. ACS Nano, 5(1):574–580, 2011.

[3] D Braun and A J Heeger. Visible-light emission from semiconducting polymer diodes. AppliedPhysics Letters, 58(18):1982–1984, MAY 6 1991.

[4] C W Tang and S A VanSlyke. Organic electroluminescent diodes. Applied Physics Letters,51(12):913–915, 1987.

[5] R H Friend, R W Gymer, A B Holmes, J H Burroughes, R N Marks, C Taliani, D D C Bradley, D ADos Santos, J L Bredas, M Logdlund, and W R Salaneck. Electroluminescence in conjugatedpolymers. Nature, 397(6715):121–128, JAN 14 1999.


[6] Hermona Christian-Pandya, Subramanian Vaidyanathan, and Mary Galvin. Polymers for use inpolymeric light-emitting diodes: structure-property relationships. In Terje A Skotheim andJohn R Reynolds, editors, Conjugated polymers - processing and applications, chapter 5. CRCPress, Boca Raton, FL, USA, 3rd edition, 2007.

[7] Qibing Pei, Gang Yu, Chi Zhang, Yang Yang, and Alan J Heeger. Polymer light-emittingelectrochemical cells. Science, 269(5227):1086–1088, 1995.

[8] Junbo Wu, Mukul Agrawal, Hector A Becerril, Zhenan Bao, Zunfeng Liu, Yongsheng Chen,and Peter Peumans. Organic light-emitting diodes on solution-processed graphene transparentelectrodes. ACS Nano, 4(1):43–48, 2010. PMID: 19902961.

[9] J W Thackeray, H S White, and M S Wrighton. Poly(3-methylthiophene)-coated electrodes —optical and electrical-properties as a function of redox potential and amplification of electricaland chemical signals using poly(3-methylthiophene)-based microelectrochemical transistors.Journal of physical chemistry, 89(23):5133–5140, 1985.

[10] Olle Inganas. Hybrid electronics and electrochemistry with conjugated polymers. Chem. Soc.Rev., 39:2633–2642, 2010.

[11] Chad M Amb, Aubrey L Dyer, and John R Reynolds. Navigating the color palette of solution-processable electrochromic polymers. Chemistry of Materials, 23(3):397–415, 2011.

[12] Pierre M Beaujuge and John R Reynolds. Color control in pi-conjugated organic polymers for usein electrochromic devices. Chemical Reviews, 110(1):268–320, 2010. PMID: 20070115.

[13] Laura Valero Conzuelo, Joaqun Arias-Pardilla, Juan V Cauich-Rodrguez, Mascha Afra Smit, andToribio Fernndez Otero. Sensing and tactile artificial muscles from reactive materials. Sensors,10(4):2638–2674, 2010.

[14] Elisabeth Smela, Wen Lu, and Benjamin R Mattes. Polyaniline actuators: Part 1. PANI(AMPS)in hcl. Synthetic Metals, 151(1):25–42, 2005.

[15] Joakim Isaksson, Carl Tengstedt, Mats Fahlman, Nathaniel Robinson, and Magnus Berggren. Asolid-state organic electronic wettability switch. Advanced Materials, 16(4):316–320, 2004.

[16] Linda Robinson, Joakim Isaksson, Nathaniel D Robinson, and Magnus Berggren. Electrochemicalcontrol of surface wettability of poly(3-alkylthiophenes). Surface Science, 600(11):L148–L152,2006.

[17] Linda Robinson, Anders Hentzell, Nathaniel D Robinson, Joakim Isaksson, and Magnus Berggren.Electrochemical wettability switches gate aqueous liquids in microfluidic systems. Lab Chip,6:1277–1278, 2006.

[18] Donna Hohertz and Jun Gao. How electrode work function affects doping and electroluminescenceof polymer light-emitting electrochemical cells. Advanced Materials, 20(17):3298–3302, 2008.

[19] Joon Ho Shin, Piotr Matyba, Nathaniel D Robinson, and Ludvig Edman. The influence ofelectrodes on the performance of light-emitting electrochemical cells. Electrochimica Acta,52(23):6456–6462, 2007.

[20] Jun Gao and Justin Dane. Planar polymer light-emitting electrochemical cells with extremelylarge interelectrode spacing. Applied Physics Letters, 83(15):3027–3029, 2003.

[21] Joon-Ho Shin, Andrzej Dzwilewski, Agnieszka Iwasiewicz, Steven Xiao, Ake Fransson,Genesis Ngwa Ankah, and Ludvig Edman. Light emission at 5 V from a polymer device with amillimeter-sized interelectrode gap. Applied Physics Letters, 89(1):013509, 2006.

[22] Nathaniel D Robinson, Joon-Ho Shin, Magnus Berggren, and Ludvig Edman. Doping frontpropagation in light-emitting electrochemical cells. Phys. Rev. B, 74(15):155210, Oct 2006.

[23] Joon-Ho Shin and Ludvig Edman. Light-emitting electrochemical cells with millimeter-sizedinterelectrode gap: Low-voltage operation at room temperature. Journal of the AmericanChemical Society, 128(49):15568–15569, 2006. PMID: 17147348.

[24] Jun Gao and Justin Dane. Visualization of electrochemical doping and light-emitting junctionformation in conjugated polymer films. Applied Physics Letters, 84(15):2778–2780, 2004.

[25] Nathaniel D Robinson, Junfeng Fang, Piotr Matyba, and Ludvig Edman. Electrochemicaldoping during light emission in polymer light-emitting electrochemical cells. Phys. Rev. B,


78(24):245202, Dec 2008.[26] Jason D Slinker, John A DeFranco, Michael J Jaquith, William R Silveira, Yu-Wu Zhong,

Jose M Moran-Mirabal, Harold G Craighead, Hector D Abruna, John A Marohn, andGeorge G Malliaras. Direct measurement of the electric-field distribution in a light-emittingelectrochemical cell. Nature Materials, 6(11):894–899, NOV 2007.

[27] Liam S C Pingree, Deanna B Rodovsky, David C Coffey, Glenn P Bartholomew, and David SGinger. Scanning kelvin probe imaging of the potential profiles in fixed and dynamic planarLECs. Journal of the American Chemical Society, 129(51):15903–15910, 2007.

[28] Piotr Matyba, Klara Maturova, Martijn Kemerink, Nathaniel D Robinson, and Ludvig Edman.The dynamic organic p-n junction. Nature Materials, 8:672–676, 2009.

[29] Zhibin Yu, Liangbing Hu, Zhitian Liu, Mingliang Sun, Meiliang Wang, George Gruner, and QibingPei. Fully bendable polymer light emitting devices with carbon nanotubes as cathode and anode.Applied Physics Letters, 95(20):203304, 2009.

[30] Masukazu Hirata, Takuya Gotou, Shigeo Horiuchi, Masahiro Fujiwara, and Michio Ohba. Thin-film particles of graphite oxide 1: High-yield synthesis and flexibility of the particles. Carbon,42(14):2929–2937, 2004.

[31] Goki Eda, Giovanni Fanchini, and Manish Chhowalla. Large-area ultrathin films of reducedgraphene oxide as a transparent and flexible electronic material. Nature Nanotechnology,3(5):270–274, MAY 2008.

[32] Hisato Yamaguchi, Goki Eda, Cecilia Mattevi, HoKwon Kim, and Manish Chhowalla. Highlyuniform 300 mm wafer-scale deposition of single and multilayered chemically derived graphenethin films. ACS Nano, 4(1):524–528, 2010. PMID: 20050640.

[33] Ludvig Edman, Daniel Moses, and Alan J Heeger. Influence of the anion on the kinetics andstability of a light-emitting electrochemical cell. Synthetic Metals, 138(3):441–446, 2003.

[34] Sukang Bae, Hyeongkeun Kim, Youngbin Lee, Xiangfan Xu, Jae-Sung Park, Yi Zheng, JayakumarBalakrishnan, Tian Lei, Hye Ri Kim, Young Il Song, Young-Jin Kim, Kwang S. Kim, BarbarosOzyilmaz, Jong-Hyun Ahn, Byung Hee Hong, and Sumio Iijima. Roll-to-roll production of 30-inch graphene films for transparent electrodes. NATURE NANOTECHNOLOGY, 5(8):574–578,AUG 2010.

[35] Andreas Sandstrom, Piotr Matyba, and Ludvig Edman. Yellow-green light-emittingelectrochemical cells with long lifetime and high efficiency. Applied Physics Letters, 96(5):053303,2010.

[36] Zhibin Yu, Meiliang Wang, Gangtie Lei, Jun Liu, Lu Li, and Qibing Pei. Stabilizing the dynamicp-i-n junction in polymer light-emitting electrochemical cells. The Journal of Physical ChemistryLetters, 2(5):367–372, 2011.

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Graphene electrodes for organic metal-free light-emitting devices

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